Guest blogger Jack Griffin is a self-described America's Cup geek, who takes pride in helping others understand and enjoy the battles of the tycoons, technologists and sailors in this quirky and fascinating event.
We've had a taste of racing wing sailed catamarans with the America's Cup World Series — preliminary regattas in AC45s. Now let's have a quick look at the AC72 catamarans that will race for the America's Cup this summer in San Francisco.
Bigger than the wing of a 747, the rigid wing sail is 130 feet (40 meters) tall and has five times the surface area of the wings that have been used to date on the AC45s. Flaps on the trailing edge provide lift. The flaps have multiple segments to shape the wing to match the wind and control the power.
Now the center of atttention, the daggerboard shapes enable hydrofoiling. Emirates Team New Zealand were the first to exploit a loophole in the design rule — no movable control surfaces are permitted on the daggerboards or rudders. But moving the entire daggerboard gives enough control to permit the AC72s to fly. The control systems are closely guarded secrets.
The "platform" provided by the hulls, crossbeams and trampoline is the size of a tennis court. You could fit six AC45s in the space of an AC72. When hydrofoiling at speeds over 40 knots, the aerodynamic drag of the hulls becomes even more important than the hydrodynamics! Of course, when the boats come off the foils, the volume in the bows is crucial to preventing a pitchpole.
Perhaps the sleeper issue in AC72 design, deck layout and crew choreography are not as sexy as wing sails and hydrofoiling. The severely short-handed crew of 11 must provide all the power for the maneuvers — no stored energy is permitted. Hoist the gennaker 130 feet to the top of the wing. Furl and unfurl it during gybes. Raise and lower the daggerboards during every tack and gybe. Trim the headsail and the wing. And sprint across the trampoline netting – 30 percent wider than a doubles tennis court – at every maneuver.
Too powerful? Too dangerous?
At the time of this writing, two of the four teams have capsized, one fatally. The other two will sail their AC72s in the winds and tides of San Francisco Bay for the first time this month. To meet the needs of TV schedules, the protocol calls for racing in as little as five knots and as much as 33 knots of wind. But there’s no way to reef the wing, so the boats are seriously overpowered above 20 knots. The safety Review Committee appointed after the death of olympic gold medalist Andrew “Bart” Simpson may recommend reducing the upper wind limit.
— Jack Griffin